Molecular genetic diversity and differentiation of Nile tilapia (Oreochromis niloticus, L. 1758) in East African natural and stocked populations

Abstract

Background: The need for enhancing the productivity of fisheries in Africa triggered the introduction of non-native fish, causing dramatic changes to local species. In East Africa, the extensive translocation of Nile tilapia (Oreochromis niloticus) is one of the major factors in this respect. Using 40 microsatellite loci with SSR-GBS techniques, we amplified a total of 664 individuals to investigate the genetic structure of O. niloticus from East Africa in comparison to Ethiopian and Burkina Faso populations.Results: All three African regions were characterized by independent gene-pools, however, the Ethiopian population from Lake Tana was genetically more divergent (F st = 2.1) than expected suggesting that it might be a different sub-species. In East Africa, the genetic structure was congruent with both geographical location and anthropogenic activities (Isolation By Distance for East Africa, R 2 = 0.67 and Uganda, R 2 = 0.24). O. niloticus from Lake Turkana (Kenya) was isolated, while in Uganda, despite populations being rather similar to each other, two main natural catchments were able to be defined. We show that these two groups contributed to the gene-pool of different non-native populations. Moreover, admixture and possible hybridization with other tilapiine species may have contributed to the genetic divergence found in some populations such as Lake Victoria. We detected other factors that might be affecting Nile tilapia genetic variation. For example, most of the populations have gone through a reduction in genetic diversity, which can be a consequence of bottleneck (G-W, < 0.5) caused by overfishing, genetic erosion due to fragmentation or founder effect resulting from stocking activities. Conclusions: The anthropogenic activities particularly in the East African O. niloticus translocations, promoted artificial admixture among Nile Tilapia populations. Translocations may also have triggered hybridization with the native congenerics, which needs to be further studied. These events may contribute to outbreeding depression and hence compromising the sustainability of the species in the region.

Genetic diversity of Nile tilapia (Oreochromis niloticus) populations in Ethiopia: insights from nuclear DNA microsatellites and implications for conservation

“…The level of differentiation in East Africa was at a similar height to the one found between East and West Africa. Our study now, adds a detailed view on genetic structure of Ethiopian samples: three major clusters divided by high F ST values exist in Ethiopia that correspond to the flowing groups from Tibihika et al [ 59 , 60 ]: (1) Lake Tana, (2) the Kenyan Rift Valley (Omo-Turkana system), and (3) the Ethiopian Rift Valley populations. Further divisions of these groups were supported by the geographical congruence of the patterns showed by the UPGMA, and STRUCTURE analysis for higher K values.…”

“…Nile tilapia population structure is highly congruent with the respective drainage basins a structure which is only intercepted by the stocked populations. In earlier studies with the same marker system implemented here [ 59 , 60 ], where only a few representatives of Ethiopian Nile tilapia were also included, five well supported clusters were identified. One contained the West African populations, while the remaining four groups corresponded to the East African populations of: Uganda, the Kenyan Rift Valley (namely Lake Turkana), Ethiopian Rift Valley and Lake Tana.…”

“…Its large natural distribution area in sub-Saharan Africa and its broad ecological tolerance makes Nile tilapia a successful species in a wide range of aquatic habitats [ 54 ]. On a macro-biogeographic scale, molecular studies have revealed clear genetic differentiation and population structure throughout its natural range [ 7 , 36 , 59 , 60 ]. According to Bezault et al [ 7 ] three major lineages of Nile tilapia are found in Africa, corresponding to (1) the Ethiopian Rift Valley (primarily the endorheic Awash River drainage), (2) the broadly defined Nilotic region (including the northern part of the Kenyan Rift Valley), and (3) the Sudano-Sahelian region in West Africa.…”

“…While most previous molecular studies [ 2 , 7 , 36 , 53 ] provided data from wide geographical areas across Africa, a more detailed investigations on genetic structure with focus on East Africa confirmed that the Nilotic region and the Ethiopian Rift Valley including adjunct waterbodies harbour several populations with rather high level of potential genetic differentiation [ 59 , 60 ]. In particular in Ethiopia substantial phylogeographic structure of Nile tilapia had been suggested by studies that include morphological observations [ 25 , 62 ].…”

“…The natural phylogeographic structure of Nile tilapia might be altered as human induced translocations of different Nile tilapia strains for aquaculture and fisheries constitute a major concern to the genetic diversity and integrity of native populations throughout Africa [ 54 , 60 ]. Besides evidence for strong genetic structure in natural populations [ 36 ], signs of admixture due to human induced translocations of non-native stocks have been documented in the upper Nile River drainage [ 58 , 60 ]. The potential threat of deliberate and uncontrolled introduction of non-native species and/or strains is well known for many freshwater fish species around the world (e.g.…”